This invention relates, in general, to quantum well devices, and more particularly, to a high mobility quantum well structure.
Semiconductor devices, such as bipolar and field effect transistors, can consist of multiple layers of semiconducting or insulating materials. Charge carriers (holes and electrons) move through the layers of the semiconductor device. Performance and efficiency of the semiconductor device is largely determined by mobility of the charge carriers in the semiconductor layers.
Bulk semiconductor materials have characteristic mobility which determines usefulness of the semiconductor material for device processing. Compound semiconductors, for example, are known to have higher mobility than silicon and thus are preferred for high performance devices. Characteristic mobility of a semiconductor material, however, is degraded by crystal imperfections, doping impurities, and the like. Mobility also decreases as a result of interactions of charge carriers with phonons of the crystal lattice.
While extensive work has been done to reduce mobility degradation caused by impurities and defects in semiconductors, it has been widely accepted that mobility degradation caused by charge carrier interaction with bulk phonons can not be reduced. In addition to phonons associated with bulk materials, interface phonons are generated at an interface between semiconductor layers of different composition. In the past, it was believed that mobility degradation caused by electron-phonon interactions was best limited by designing structures with as few interfaces, and thus as few semiconductor layers as possible.
In a bulk material electrons and phonons are distributed uniformly. Interaction between the two is proportional to the product of the densities and it is thus constant throughout the sample. What is needed is a semiconductor material structure which separates charge carrier and phonon distributions, reducing electron-phonon interaction and improving mobility.